The present invention relates generally to optical fibers and, more particularly, to optical fibers in which the occurrence of four-wave mixing is reduced.
The advent of wavelength-division multiplexing (WDM) in optical networking has increased the demands placed on optical fibers. Manufacturers are pressured to produce optical fibers that are capable of carrying optical signals on a wider range of wavelengths and over longer distances with less loss than can be accomplished with presently available optical fibers. Concurrent developments in optical sources for WDM have increased the amount of optical power and bit rates that must be transmitted through the optical fiber.
The demand for higher optical power throughput and wider range of transmitted wavelengths has led to an increase in the occurrence of error due to nonlinear optical effects produced during light propagation through the optical fiber. Four-wave mixing (FWM) is especially problematic in WDM networks because this nonlinear optical effect leads communication errors that cannot be easily removed by known solutions such as, for example, wavelength filtering equalization.[1]
As is well-known in the art, FWM is the induced combination of three wavelengths to produce one or more new lengths. Two of the three combining wavelengths can be degenerate such that FWM also includes the combination of two wavelengths to produce one or more new wavelengths. Optical power is taken away from the combining wavelengths and transferred to the new wavelengths in the FWM process. FWM is especially problematic in optical communications if the new wavelengths produced by the FWM process overlap the assigned wavelengths of existing WDM channels because it is difficult to distinguish between the legitimate optical data signals at these existing WDM channels and the error signal superimposed thereon as the result of FWM. Therefore, it is of particular concern to the practitioner of the art to suppress FWM.
It is also well-known that the efficiency of the FWM process increases when the potentially combining wavelengths travel along the optical fiber at the same group velocity over an extended distance. In other words, the longer the potentially combining wavelengths travel together down the optical fiber at the same group velocity, the higher the risk of communication error resulting from FWM. A typical group index profile, generally indicated by reference number 10, as a function of wavelength is shown in FIG. 1. As shown in FIG. 1, a conventional optical fiber typically exhibits a group index profile that is slightly parabolic in shape over the optical fiber communication wavelength range. As indicated by dotted lines 12, 14 and 16, there exist pairs of wavelengths, such as λ1 and λ2 shown in FIG. 1, that share the same group index values. In the example shown in FIG. 1, light signal of wavelength λ1 and light signal of wavelength λ2 traveling through the optical fiber will both “see” a group index value of n1=ng(λ1)=ng (λ2). Since the group velocity is related to the group index by the equation: vg=c/ng, the group velocity of a light signal of wavelength λ1 is equal to the group velocity of a light signal of wavelength λ2. In this way, in conventional optical fibers, the group velocities of the shorter wavelengths in the wavelength range are generally the same as the group velocities of the longer wavelengths in the wavelength range. As a result, the FWM efficiency for the combination of the shorter wavelengths and the longer wavelengths in the optical fiber communication wavelength range is high, therefore leading to a high probability of potential error introduced in the transmitted optical signals. In the example shown in FIG. 1, the FWM efficiency for the combination of λ1 with λ2 is high, therefore leading to a high probability of potential error occurring due to FWM.
Advances in chromatic dispersion shifting and reduction in optical fibers have actually exacerbated the problem because FWM efficiency increases around the wavelength at which chromatic dispersion is zero. Increased optical power at the potentially combining wavelengths also increases the FWM efficiency. Furthermore, increased variety of wavelengths used at the WDM channels also exacerbate the FWM problem because more optical wavelength combinations are available for the FWM process.
One known way to prevent the occurrence of FWM is to keep the optical power throughput low over the operating wavelength range of the WDM network. However, reduction of optical power leads to problems such as the cost associated with the need for additional repeaters to regenerate the optical signals and the potential increase in bit error rates due to signal weakness. The technological demand for increased distance between repeaters, reduced cost and more reliable data transmission makes this approach impractical.
Another approach to FWM suppression is to provide small but non-zero chromatic dispersion over the operating wavelength range. By introducing variation in the group velocities at different wavelengths in this way, FWM efficiency is reduced. This approach may be implemented using specialty optical fibers known in the art such as, for example, non-zero dispersion fiber (NZDF) and non-zero dispersion-shifted fiber (NZ-DSF). A dispersion compensation fiber (DCF) may also be used in this approach. Also known as a negative dispersion fiber, DCF is generally an optical fiber whose chromatic dispersion decreases with increased wavelength (negative dispersion), as opposed to most other optical fibers whose chromatic dispersion increases at longer wavelengths (positive dispersion). By splicing predetermined lengths of DCF into an optical network that has been implemented using positive dispersion optical fiber, the overall chromatic dispersion profile of the network can be manipulated in such a way that a small but non-zero chromatic dispersion is present across the operating wavelength range thus suppressing FWM.
There exist commercially-available optical fibers which combine the two aforedescribed approaches to FWM suppression. The LEAF fiber (which is an abbreviation for Large Effective Area Fiber), available from Corning, provides a larger effective mode area, in comparison to most available NZ-DSF, through which the optical signals travel.[2] Thus, since the optical power is spread over a larger area of the optical fiber than in conventional fibers, more optical power can be directed down the optical fiber without inducing nonlinear effects such as FWM. In addition, the LEAF has the characteristics of NZ-DSF such that FWM efficiency is further reduced. The TrueWave XL fiber, manufactured by Lucent Technologies, also features a larger effective modal area as well as negative dispersion properties.
A problem common to the aforedescribed prior art approaches is the manufacturing complexity of the specialty optical fibers. Careful design and precision fabrication are required in order to achieve the often complex, radial refractive index profiles of these specialty fibers. The transmission characteristics of conventional optical fibers are established simply by the radius of the core and the relative values of the core refractive index and the cladding refractive index. However, the specialty optical fibers for FWM suppression require refractive index profiles that may vary linearly, parabolically, in steps or some combination thereof in the radial direction from the center of the fiber core. Some specialty fibers even require multiple cladding layers of different materials. The design and fabrication of these speciality fibers can be complicated and costly.[3]
The present invention provides an optical fiber which serves to resolve the problems described above with regard to prior art optical fibers in a heretofore unseen and highly advantageous way and which provides still further advantages.